Editing Keeling Curve (section)

===Passing 400 ppm in 2013===
On May 9, 2013, the daily mean concentration of {{CO2}} in the atmosphere measured at Mauna Loa surpassed 400 parts per million ([[Parts-per notation|ppm]]).<ref>{{Cite journal|last=Showstack|first=Randy|date=2013|title=Carbon dioxide tops 400 ppm at Mauna Loa, Hawaii|journal=Eos, Transactions American Geophysical Union|volume=94|issue=21|pages=192|doi=10.1002/2013eo210004|bibcode=2013EOSTr..94Q.192S|issn=0096-3941|doi-access=free}}</ref> Estimates of {{CO2}} during previous [[geologic era]]s suggest that {{CO2}} has not reached this level since the [[Pliocene climate|mid-Pliocene]], 2 to 4 million years ago.<ref name="Montaigne">{{cite web|url=http://e360.yale.edu/feature/keeling_curve_son_of_climate_science_pioneer_on_co2_milestone/2650/|title=Son of Climate Science Pioneer Ponders A Sobering Milestone|last1=Montaigne|first1=Fen|website=Yale Environment 360|publisher=Yale School of Forestry & Environmental Studies|access-date=14 May 2013|archive-url=https://web.archive.org/web/20130608174549/http://e360.yale.edu/feature/keeling_curve_son_of_climate_science_pioneer_on_co2_milestone/2650/|archive-date=8 June 2013|url-status=dead}}</ref> This level of carbon dioxide, causing [[climate change]], suggests a continued worsening in natural and ecological disasters, which increasingly threatens human and animal habitats on Earth, if greenhouse gas emissions are not significantly reduced.